The effects of oral melengestrol acetate (mga) on ovarian activity have been studied in 264 dairy and beef heifers. A total of ninety-eight heifers received 0·4 mg of mga daily for short periods of time (18 to 32 days) and ovarian activity was determined by rectal palpation. As the incidence of a detectable corpus luteum decreased from 76 % to 10 %, the incidence of a detectable follicle increased from 56 % to 91 %. Follicular size also increased with time. Samples of cervical mucus were rated according to fern pattern, which was interpreted as due to an oestrogenic influence, and which occurred upon regression of the corpus luteum even though oestrus and ovulation were inhibited by mga treatment.
A total of 166 beef heifers was used in long-term studies (105 to 116 days) of mga treatment. Significant (P<0·05) increases were found in the weight of follicular fluid, primarily of the largest follicle from the pair of ovaries. Also indicative of increased oestrogenicity was an increased adrenal weight in puberal, intact mga-treated heifers. The adrenal weights of spayed heifers tended to decrease with mga treatment. The optimal dose for increased follicular activity appears to be near 0·4 mg mga daily/heifer.